1. Field of the Invention
The present invention relates to an outboard motor mounted in a boat.
2. Description of the Related Art
In general, an outboard motor mounted in a boat has a cowling, an upper casing provided below the cowling, and a lower casing disposed below the upper casing.
An engine is disposed in the cowling. An exhaust passage connected to a cylinder of the engine is arranged to extend through the cowling, the upper casing, and the lower casing to a lower portion of the lower casing. The exhaust passage is provided with a catalyst that purifies exhaust gas.
In such a construction, exhaust gas that has flowed out from each cylinder to the exhaust passage is purified in the catalyst, and then discharged into water from a bottom portion of the lower casing.
A lower end portion of the exhaust passage is immersed in water. Therefore, water in a lower end portion of the exhaust passage may flow backward to an engine side as a result of negative pressure or the like that is generated in the engine. Especially, a four-cycle engine is largely affected by exhaust pulsation, so water is sucked to an engine side by strong force, in the exhaust passage.
In order to prevent deterioration of the catalyst, water flowing backward in the exhaust passage must be prevented from adhering to the catalyst. To prevent water adhesion to a catalyst, an outboard motor in which a catalyst is disposed in a cowling has been developed (for example, refer to JP-A-2000-356123).
However, when the catalyst is disposed in the cowling, the temperature in the cowling rises due to heat radiated from the catalyst. Accordingly, electronic components (rectifier regulator, etc.) disposed in the cowling and fuel system components (vapor separator tank, etc.) may suffer from malfunction caused by the increase in heat. Also, heat resistance of the components in the cowling must be improved. Thus, a product cost of outboard motor rises.